A conventional ETFE copolymer is well known as an alternating copolymer having good cut-through resistance, melt-moldability, chemical resistance and electrical properties. However, it is highly crystalline and fragile at a high temperature. To overcome this drawback of the ETFE copolymer, it is proposed to copolymerize a third monomer as a modifier with ethylene and tetrafluoroethylene. For example, U.S. Pat. No. 3,624,250 discloses copolymerization of a vinyl monomer having no telogenic activity and providing the copolymer with side chains having at least two carbon atoms as the third monomer with ethylene and tetrafluoroethylene. However, as described on column 3, lines 36-44 of the specification of said patent, when the modifier vinyl monomer is copolymerized, the molar ratio of ethylene and tetrafluoroethylene should be from 60:40 to 40:60. Otherwise, tensile properties and cut-through resistance of the copolymer are unfavorably deteriorated. From this description, it is understood that the invention of said U.S. Patent intends to improve mechanical properties of the alternating copolymer at a high temperature while keeping the characteristics of the alternating copolymer as much as possible.
ETFE copolymer is required to have many diverse properties. For example, in case where it is used as a material of a laminate film with a rubber, it should be chemical resistant and flexible, that is, it should have low flexural modulus and small yield strength, so that it does not deteriorate low flexural modulus of the rubber. When a plug or stopper for a chemical vial is coated with such laminate film, sealing performance is improved due to its flexibility. Further, a flame resistant material is highly desired, for instance, as a coating material of a plenum cable. For example, when the conventional ETFE copolymer and polyvinylidene fluoride, which have substantially the same elementary composition, are examined for oxygen index, it is 30% for the former while 43% for the latter. Therefore, it is desired to improve ETFE copolymer to have a oxygen index not smaller than that of polyvinylidene fluoride. ride.